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On 25/10/2013 05:40, John Chester wrote:
> At 10:51 PM 10/24/2013, Malcolm Rockwell wrote:
>> There may not be a major problem here. What has printed through is 
>> the audio from the next layer of tape, correct? With digital 
>> manipulation being what it is today it should be simple enough to 
>> grab the full volume layer of audio, attenuate it, flip the waveform 
>> and apply it "over" the printed through signal. There will probably 
>> be artifacts but if you fiddle with various parameters for a while, 
>> such as EQ, you will probably be able to find an acceptable solution 
>> to your problem. I'd apply this to softer passages and leave louder 
>> material well enough alone, though.
>> It's worth a try.
>> Comments?
>
> There are a number of problems to consider.
>
> First, the printed recording is not the same length as the original.  
> The delay between original and print changes as tape pack diameter 
> changes.  Seems to me that for a tape which has been stored tails out, 
> the print is longer than the original.  (Delay from original to print 
> increases as tape pack diameter on the take up reel increases.)  The 
> original recording can be speed-shifted, but you need to figure out 
> how much to shift it.
>
> Second, the frequency response of the printing process is not flat.  
> According to
> http://www.aes.org/aeshc/docs/3mtape/printthrough.pdf
> "The worst print-through occurs at a wavelength equal to 27 * C. C is 
> the total tape caliper in mils. For a typical 2
> mil mastering tape, the worst wavelength for print-through would be 
> about 12.6 mils. When running at 15 ips, this
> would be a frequency of about 1200 Hz."
> Note:  there's an error in their formula, as printed in the on-line 
> document.  It should read 2 * Pi * C.  But they're correct that it's 
> worst at about 1200 Hz.  This is confirmed by the October 1980 JAES 
> article "The Print-Through Phenomenon" by Bertram, Stafford and 
> Mills.  It includes a graph of print-through vs. frequency.
>
> BTW, this article also states that "print-through ... can be reduced 
> if [the tape] is repeatedly rewound.  The amount of print reduction 
> ...can reach as much as 7 dB."  In their tests, this required 6 
> rewindings.  "The rewindings should be consecutive with an optimum 
> storage time between rewindings to achieve maximum reduction.  The 
> optimum storage time may depend upon the individual tape."
>
> Third, is the printing process linear or non-linear?  The 3M document 
> cited above says it's linear.  Camras, in the 1988 edition of 
> "Magnetic Recording Handbook", says it's not, and that the ratio of 
> the original to printed signal varies with the level of the original 
> signal.  I think Bertram et al.  are saying it is linear, but I must 
> admit that I have not yet entirely digested this long, complex 
> article.  Hopefully it is linear, because modelling a non-linear 
> transfer function will not be much fun.
>
> So....  If the printing process is linear, the other problems seem 
> manageable.  But it will no doubt require a fair bit of fiddling to 
> get the cancellation signal lined up in time and amplitude with each 
> objectionable echo.
>
> -- John Chester
>
I routinely use Cedar Retouch for print-through problems, whether pre- 
or post-echo, and usually find that removing the two or three most 
prominent components is enough to push it back into the noise. To use 
agressive and irreversible techniques on an analogue original is surely 
to be discouraged when other techniques are available.